To analyze the correlation between phenotype/genotype to selected antiretroviral agents and short-term change in viral load upon discontinuation of a single antiretroviral agent from a failing regimen, one drug in a failing multidrug regimen is withdrawn for a limited period of time, and then restored. By monitoring changes in both viremia and genotype (by the limiting dilution assay), we can discern whether the drug was contributing to partial suppression of virus, and also determine which mutations are associated with resistance to that drug, and their effect on both replication of the virus and resistance to the drug. In collaboration with Dr. Vinay Pathak, we have begun to analyze the entire RT sequence to determine whether additional changes in the RNAse H portion of RT are associated with partial drug suppression. With Dr. Anuradha Ganesan, we have initiated a new site for this protocol at the National Naval Medical Center. [Corresponds to Project 4 in the April 2007 site visit report of the Host-Virus Interaction Unit, HIV Drug Resistance Program]